[SPARK-24709][SQL][2.4] use str instead of basestring in isinstance

## What changes were proposed in this pull request?

after backport apache#22775 to 2.4, the 2.4 sbt Jenkins QA job is broken, see https://amplab.cs.berkeley.edu/jenkins/view/Spark%20QA%20Test/job/spark-branch-2.4-test-sbt-hadoop-2.7/147/console

This PR adds `if sys.version >= '3': basestring = str` which onlly exists in master.

## How was this patch tested?

existing test

Closes apache#22858 from cloud-fan/python.

Authored-by: Wenchen Fan <[email protected]>
Signed-off-by: hyukjinkwon <[email protected]>

Author: cloud-fan

python/pyspark/sql/functions.py

@@ -25,6 +25,9 @@
 if sys.version < "3":
     from itertools import imap as map
 
+if sys.version >= '3':
+    basestring = str
+
 from pyspark import since, SparkContext
 from pyspark.rdd import ignore_unicode_prefix, PythonEvalType
 from pyspark.sql.column import Column, _to_java_column, _to_seq, _create_column_from_literal